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<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="initialization.html" title="cpp/language/initialization"> Initialization</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> <a href="initialization.html" title="cpp/language/initialization"> Initializer </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_initialization.html" title="cpp/language/default initialization"> Default initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="value_initialization.html" title="cpp/language/value initialization"> Value initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="direct_initialization.html" title="cpp/language/direct initialization"> Direct initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_initialization.html" title="cpp/language/copy initialization">Copy initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="list_initialization.html" title="cpp/language/list initialization">List initialization</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">Aggregate initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">Reference initialization</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_elision.html" title="cpp/language/copy elision">Copy elision</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> <a href="initialization.html#Static_initialization" title="cpp/language/initialization"> Static initialization </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="zero_initialization.html" title="cpp/language/zero initialization">Zero initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="constant_initialization.html" title="cpp/language/constant initialization">Constant initialization</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> <a href="initialization.html#Dynamic_initialization" title="cpp/language/initialization"> Dynamic non-local initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initialization.html#Dynamic_initialization" title="cpp/language/initialization"> Ordered dynamic initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initialization.html#Dynamic_initialization" title="cpp/language/initialization">Unordered dynamic initialization</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class member initialization </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">Member initializer list</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">Default member initializer</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Binds a reference to an object.
</p>
<h3><span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> 
<p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>=</b></code> <span class="t-spar">target</span> <code><b>;</b></code>
</p><p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>= {</b></code> <span class="t-spar">arg1</span><code><b>,</b></code> <span class="t-spar">arg2</span><code><b>,</b></code> ... <code><b>};</b></code>
</p><p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>(</b></code> <span class="t-spar">target</span> <code><b>);</b></code>
</p><p><span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>{</b></code> <span class="t-spar">arg1</span><code><b>,</b></code> <span class="t-spar">arg2</span><code><b>,</b></code> ... <code><b>};</b></code>
</p>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> 
<p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>=</b></code> <span class="t-spar">target</span> <code><b>;</b></code>
</p><p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>= {</b></code> <span class="t-spar">arg1</span><code><b>,</b></code> <span class="t-spar">arg2</span><code><b>,</b></code> ... <code><b>};</b></code>
</p><p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>(</b></code> <span class="t-spar">target</span> <code><b>);</b></code>
</p><p><span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>{</b></code> <span class="t-spar">arg1</span><code><b>,</b></code> <span class="t-spar">arg2</span><code><b>,</b></code> ... <code><b>};</b></code>
</p>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> given <span class="t-spar">R</span> <span class="t-spar">fn</span> <code><b>(</b></code> <span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">arg</span> <code><b>);</b></code><span class="t-rev-inl t-since-cxx11"><span> or <span class="t-spar">R</span> <span class="t-spar">fn</span> <code><b>(</b></code> <span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">arg</span> <code><b>);</b></code></span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>
<p><span class="t-spar">fn</span> <code><b>(</b></code> <span class="t-spar">target</span> <code><b>)</b></code>
</p><p><span class="t-spar">fn</span> <code><b>({</b></code> <span class="t-spar">arg1</span><code><b>,</b></code> <span class="t-spar">arg2</span><code><b>,</b></code> ... <code><b>});</b></code>
</p>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> inside <span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">fn</span> <code><b>()</b></code><span class="t-rev-inl t-since-cxx11"><span> or <span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">fn</span> <code><b>()</b></code></span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>
<p><code><b>return</b></code> <span class="t-spar">target</span> <code><b>;</b></code>
</p>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> given <span class="t-spar">T</span> <code><b>&amp;</b></code> <span class="t-spar">ref</span> <code><b>;</b></code><span class="t-rev-inl t-since-cxx11"><span> or <span class="t-spar">T</span> <code><b>&amp;&amp;</b></code> <span class="t-spar">ref</span> <code><b>;</b></code></span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> inside the definition of <span class="t-spar">Class</span>
<p><span class="t-spar">Class</span><code><b>::</b></code><span class="t-spar">Class</span><code><b>(</b></code>...<code><b>) :</b></code> <span class="t-spar">ref</span> <code><b>(</b></code> <span class="t-spar">target</span> <code><b>) {</b></code> ... <code><b>}</b></code>
</p>
</td>
<td> (5)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>A reference to <code>T</code> can be initialized with an object of type <code>T</code>, a function of type <code>T</code>, or an object implicitly convertible to <code>T</code>. Once initialized, a reference cannot be changed to refer to another object.
</p><p>References are initialized in the following situations:
</p>
<div class="t-li1"><span class="t-li">1)</span> When a named <a href="reference.html#Lvalue_references" title="cpp/language/reference">lvalue reference</a> variable is declared with an initializer</div>
<div class="t-li1"><span class="t-li">2)</span> When a named <a href="reference.html#Rvalue_references" title="cpp/language/reference">rvalue reference</a> variable is declared with an initializer</div>
<div class="t-li1"><span class="t-li">3)</span> In a function call expression, when the function parameter has reference type</div>
<div class="t-li1"><span class="t-li">4)</span> In the <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span></span></span> statement, when the function returns a reference type</div>
<div class="t-li1"><span class="t-li">5)</span> When a <a href="data_members.html" title="cpp/language/data members">non-static data member</a> of reference type is initialized using a <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">member initializer</a></div>
<p>The effects of reference initialization are:
</p>
<ul><li> If the initializer is a braced-init-list ( <code><b>{</b></code> <span class="t-spar">arg1</span><code><b>,</b></code> <span class="t-spar">arg2</span><code><b>,</b></code> ... <code><b>}</b></code> ), rules of <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a> are followed.
</li></ul>
<ul><li> Otherwise, if the reference is an lvalue reference:
</li></ul>
<dl><dd><ul><li> If <span class="t-spar">target</span> is an lvalue expression, and its type is <code>T</code> or derived from <code>T</code>, and is equally or less cv-qualified, then the reference is bound to the object identified by the lvalue or to its base class subobject.
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu16">2.0</span><span class="sy4">;</span>
<span class="kw4">double</span><span class="sy3">&amp;</span> rd <span class="sy1">=</span> d<span class="sy4">;</span>        <span class="co1">// rd refers to d</span>
<span class="kw4">const</span> <span class="kw4">double</span><span class="sy3">&amp;</span> rcd <span class="sy1">=</span> d<span class="sy4">;</span> <span class="co1">// rcd refers to d</span>
 
<span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span><span class="br0">}</span> b<span class="sy4">;</span>
 
A<span class="sy3">&amp;</span> ra <span class="sy1">=</span> b<span class="sy4">;</span>             <span class="co1">// ra refers to A subobject in b</span>
<span class="kw4">const</span> A<span class="sy3">&amp;</span> rca <span class="sy1">=</span> b<span class="sy4">;</span>      <span class="co1">// rca refers to A subobject in b</span></pre></div></div>
<dl><dd><ul><li> Otherwise, if the type of <span class="t-spar">target</span> is not same or derived from <code>T</code>, and <span class="t-spar">target</span> has conversion function to an lvalue whose type is either <code>T</code> or derived from <code>T</code>, equally or less cv-qualified,  then the reference is bound to the object identified by the lvalue returned by the conversion function (or to its base class subobject).
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span> operator <span class="kw4">int</span><span class="sy3">&amp;</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span><span class="sy3">&amp;</span> ir <span class="sy1">=</span> B<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ir refers to the result of B::operator int&amp;</span></pre></div></div>
<ul><li> Otherwise, if the reference is lvalue reference to a non-volatile const-qualified type <span class="t-rev-inl t-since-cxx11"><span> or rvalue reference</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>:
</li></ul>
<dl><dd><ul><li> If <span class="t-spar">target</span> is a non-bit-field rvalue or a function lvalue, and its type is either <code>T</code> or derived from <code>T</code>, equally or less cv-qualified, then the reference is bound to the value of the initializer expression or to its base subobject <span class="t-rev-inl t-since-cxx17"><span>(after <a href="implicit_cast.html#Temporary_materialization" title="cpp/language/implicit conversion">materializing a temporary</a> if necessary)</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>.
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">extern</span> B f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">const</span> A<span class="sy3">&amp;</span> rca2 <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// bound to the A subobject of the B rvalue.</span>
A<span class="sy3">&amp;&amp;</span> rra <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// same as above</span>
 
<span class="kw4">int</span> i2 <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>
<span class="kw4">int</span><span class="sy3">&amp;&amp;</span> rri <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span>i2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// bound directly to i2</span></pre></div></div>
<dl><dd><ul><li> Otherwise, if the type of <span class="t-spar">target</span> is not same or derived from <code>T</code>, and <span class="t-spar">target</span> has conversion function to a rvalue or a function lvalue whose type is either <code>T</code> or derived from <code>T</code>, equally or less cv-qualified,  then the reference is bound to the result of the conversion function or to its base class subobject <span class="t-rev-inl t-since-cxx17"><span>(after <a href="implicit_cast.html#Temporary_materialization" title="cpp/language/implicit conversion">materializing a temporary</a> if necessary)</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>.
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> X <span class="br0">{</span> operator B<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> x<span class="sy4">;</span>
 
<span class="kw4">const</span> A<span class="sy3">&amp;</span> r <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// bound to the A subobject of the result of the conversion</span>
B<span class="sy3">&amp;&amp;</span> rrb <span class="sy1">=</span> x<span class="sy4">;</span>    <span class="co1">// bound directly to the result of the conversion</span></pre></div></div>
<dl><dd><ul><li> Otherwise, <span class="t-spar">target</span> is implicitly converted to <code>T</code>. The reference is bound to the result of the conversion<span class="t-rev-inl t-since-cxx17"><span> (after <a href="implicit_cast.html#Temporary_materialization" title="cpp/language/implicit conversion">materializing a temporary</a>, and the cv-qualification of <code>T</code> is preserved even if it is a scalar type)</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>. If the <span class="t-spar">target</span> (or, if the conversion is done by user-defined conversion, the result of the conversion function) is of type <code>T</code> or derived from <code>T</code>, it must be equally or less cv-qualified than <code>T</code><span class="t-rev-inl t-since-cxx11"><span>, and, if the reference is an rvalue reference, must not be an lvalue</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> rs <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// rs refers to temporary copy-initialized from char array</span>
<span class="kw4">const</span> <span class="kw4">double</span><span class="sy3">&amp;</span> rcd2 <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>        <span class="co1">// rcd2 refers to temporary with value 2.0</span>
<span class="kw4">int</span> i3 <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="kw4">double</span><span class="sy3">&amp;&amp;</span> rrd3 <span class="sy1">=</span> i3<span class="sy4">;</span>            <span class="co1">// rrd3 refers to temporary with value 2.0</span></pre></div></div>
<h3><span class="mw-headline" id="Lifetime_of_a_temporary">Lifetime of a temporary</span></h3>
<p>Whenever a reference is bound to a temporary object or to a subobject thereof, the lifetime of the temporary object is extended to match the lifetime of the reference (check <a href="lifetime.html#Temporary_object_lifetime" title="cpp/language/lifetime">temporary object lifetime exceptions</a>), where the temporary object or its subobject is denoted by one of following expression:
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx17"><td>
<ul><li> a <a href="value_category.html#prvalue" title="cpp/language/value category">prvalue</a> expression of an object type,
</li></ul>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
<tr class="t-rev t-since-cxx17"><td>
<ul><li> a <a href="implicit_cast.html#Temporary_materialization" title="cpp/language/implicit conversion">temporary materialization conversion</a>,
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<ul><li> a parenthesized expression <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>e<span class="br0">)</span></span></span>, where <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> is one of these expressions,
</li><li> a <a href="operator_member_access.html#Built-in_subscript_operator" title="cpp/language/operator member access">built-in subscript expression</a> of form <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">[</span>n<span class="br0">]</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">n<span class="br0">[</span>a<span class="br0">]</span></span></span>, where <span class="t-c"><span class="mw-geshi cpp source-cpp">a</span></span> is an array and is one of these expressions,
</li><li> a <a href="operator_member_access.html#Built-in_member_access_operators" title="cpp/language/operator member access">class member access expression</a> of form <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">m</span></span></span>, where <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> is one of these expressions and <span class="t-c"><span class="mw-geshi cpp source-cpp">m</span></span> designates a non-static data member of object type,
</li><li> a <a href="operator_member_access.html#Built-in_pointer-to-member_access_operators" title="cpp/language/operator member access">pointer-to-member operation</a> of form <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="sy2">*</span>mp</span></span>, where <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> is one of these expressions and <span class="t-c"><span class="mw-geshi cpp source-cpp">mp</span></span> is a pointer to data member,
</li><li> a <a href="const_cast.html" title="cpp/language/const cast"><code>const_cast</code></a>, <a href="static_cast.html" title="cpp/language/static cast"><code>static_cast</code></a>, <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><code>dynamic_cast</code></a>, or <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><code>reinterpret_cast</code></a> conversion without a user-defined conversion that converts one of these expressions to the glvalue refers to the object designated by the operand, or to its complete object or a subobject thereof (an <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit cast</a> expression is interpreted as a sequence of these casts),
</li><li> a <a href="operator_other.html#Conditional_operator" title="cpp/language/operator other">conditional expression</a> of form <span class="t-c"><span class="mw-geshi cpp source-cpp">cond <span class="sy4">?</span> e1 <span class="sy4">:</span> e2</span></span> that is a glvalue, where <span class="t-c"><span class="mw-geshi cpp source-cpp">e1</span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">e2</span></span> is one of these expressions, or
</li><li> a <a href="operator_other.html#Built-in_comma_operator" title="cpp/language/operator other">built-in comma expression</a> of form <span class="t-c"><span class="mw-geshi cpp source-cpp">x, e</span></span> that is a glvalue, where <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> is one of these expressions.
</li></ul>
<p>There are following exceptions to this lifetime rule:
</p>
<ul><li> a temporary bound to a return value of a function in a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span></span></span> statement is not extended: it is destroyed immediately at the end of the return expression. Such <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span></span></span> statement always returns a dangling reference.
</li><li> a temporary bound to a reference parameter in a function call exists until the end of the full expression containing that function call: if the function returns a reference, which outlives the full expression, it becomes a dangling reference.
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> a temporary bound to a reference in the initializer used in a new-expression exists until the end of the full expression containing that new-expression, not as long as the initialized object. If the initialized object outlives the full expression, its reference member becomes a dangling reference.
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<ul><li> a temporary bound to a reference in a reference element of an aggregate initialized using <a href="direct_initialization.html" title="cpp/language/direct initialization">direct-initialization</a> syntax <b>(</b>parentheses<b>)</b> exists until the end of the full expression containing the initializer, as opposed to <a href="list_initialization.html" title="cpp/language/list initialization">list-initialization</a> syntax <b>{</b>braces<b>}</b>.
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A
<span class="br0">{</span>
    <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> r<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
A a1<span class="br0">{</span><span class="nu0">7</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK, lifetime is extended</span>
A a2<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// well-formed, but dangling reference</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<p>In general, the lifetime of a temporary cannot be further extended by "passing it on": a second reference, initialized from the reference variable or data member to which the temporary was bound, does not affect its lifetime.
</p>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>References appear without initializers only in function parameter declaration, in function return type declaration, in the declaration of a class member, and with the <a href="storage_duration.html" title="cpp/language/storage duration"><code>extern</code></a> specifier.
</p><p>Until the resolution of <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1696.html">CWG issue 1696</a>, a temporary is permitted to bound to a reference member in a constructor <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">initializer list</a>, and it persists only until the constructor exits, not as long as the object exists. Such initialization is ill-formed since <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1696.html">CWG  1696</a>, although many compilers still support it (a notable exception is clang).
</p>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;utility&gt;</span>
<span class="co2">#include &lt;sstream&gt;</span>
 
<span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">int</span> mi<span class="sy4">;</span>
    <span class="kw4">const</span> <a href="../utility/pair.html"><span class="kw1099">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy3">&amp;</span> mp<span class="sy4">;</span> <span class="co1">// reference member</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">void</span> foo<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
 
<span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B <span class="sy4">:</span> A
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>
    operator <span class="kw4">int</span><span class="sy3">&amp;</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
B bar<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> B<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">//int&amp; bad_r;      // error: no initializer</span>
<span class="kw4">extern</span> <span class="kw4">int</span><span class="sy3">&amp;</span> ext_r<span class="sy4">;</span> <span class="co1">// OK</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">//  Lvalues</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r1 <span class="sy1">=</span> n<span class="sy4">;</span>                    <span class="co1">// lvalue reference to the object n</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> cr<span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// reference can be more cv-qualified</span>
    <span class="kw4">volatile</span> <span class="kw4">int</span><span class="sy3">&amp;</span> cv<span class="br0">{</span>n<span class="br0">}</span><span class="sy4">;</span>            <span class="co1">// any initializer syntax can be used</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r2 <span class="sy1">=</span> r1<span class="sy4">;</span>                   <span class="co1">// another lvalue reference to the object n</span>
<span class="co1">//  int&amp; bad = cr;                  // error: less cv-qualified</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r3 <span class="sy1">=</span> <span class="kw1">const_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span>cr<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// const_cast is needed</span>
 
    <span class="kw4">void</span> <span class="br0">(</span><span class="sy3">&amp;</span>rf<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> foo<span class="sy4">;</span> <span class="co1">// lvalue reference to function</span>
    <span class="kw4">int</span> ar<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span>
    <span class="kw4">int</span> <span class="br0">(</span><span class="sy3">&amp;</span>ra<span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> ar<span class="sy4">;</span>     <span class="co1">// lvalue reference to array</span>
 
    B b<span class="sy4">;</span>
    A<span class="sy3">&amp;</span> base_ref <span class="sy1">=</span> b<span class="sy4">;</span>        <span class="co1">// reference to base subobject</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> converted_ref <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// reference to the result of a conversion</span>
 
<span class="co1">//  Rvalues</span>
<span class="co1">//  int&amp; bad = 1;        // error: cannot bind lvalue ref to rvalue</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> cref <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// bound to rvalue</span>
    <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> rref <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>      <span class="co1">// bound to rvalue</span>
 
    <span class="kw4">const</span> A<span class="sy3">&amp;</span> cref2 <span class="sy1">=</span> bar<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// reference to A subobject of B temporary</span>
    A<span class="sy3">&amp;&amp;</span> rref2 <span class="sy1">=</span> bar<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>      <span class="co1">// same</span>
 
    <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> xref <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// bind directly to n</span>
<span class="co1">//  int&amp;&amp; copy_ref = n;                 // error: can't bind to an lvalue</span>
    <span class="kw4">double</span><span class="sy3">&amp;&amp;</span> copy_ref <span class="sy1">=</span> n<span class="sy4">;</span>              <span class="co1">// bind to an rvalue temporary with value 1.0</span>
 
<span class="co1">//  Restrictions on temporary lifetimes</span>
    <a href="../io/basic_ostream.html"><span class="kw1733">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> buf_ref <span class="sy1">=</span> <a href="../io/basic_ostringstream.html"><span class="kw1740">std::<span class="me2">ostringstream</span></span></a><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'a'</span><span class="sy4">;</span> <span class="co1">// the ostringstream temporary</span>
                      <span class="co1">// was bound to the left operand of operator&lt;&lt;</span>
                      <span class="co1">// but its lifetime ended at the semicolon</span>
                      <span class="co1">// so buf_ref is a dangling reference</span>
 
    S a <span class="br0">{</span><span class="nu0">1</span>, <span class="br0">{</span><span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span>         <span class="co1">// temporary pair {2, 3} bound to the reference member</span>
                             <span class="co1">// a.mp and its lifetime is extended to match </span>
                             <span class="co1">// the lifetime of object a</span>
    S<span class="sy2">*</span> p <span class="sy1">=</span> new S<span class="br0">{</span><span class="nu0">1</span>, <span class="br0">{</span><span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// temporary pair {2, 3} bound to the reference</span>
                             <span class="co1">// member p-&gt;mp, but its lifetime ended at the semicolon</span>
                             <span class="co1">// p-&gt;mp is a dangling reference</span>
    delete p<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/391.html">CWG 391</a>
</td>
<td> C++98
</td>
<td> initialize a reference to const-qualified type with a class type<br>rvalue might create a temporary, and a constructor of that class<br>was required in order to copy the rvalue into that temporary
</td>
<td> no temporary is<br>created, constructor<br>is not required
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/450.html">CWG 450</a>
</td>
<td> C++98
</td>
<td> a reference to const-qualified array could not be<br>initialized with a reference-compatible array rvalue
</td>
<td> allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/656.html">CWG 656</a>
</td>
<td> C++98
</td>
<td> a reference to const-qualified type initialized with a type which is not<br>reference-compatible but has a conversion function to a reference-<br>compatible type was bound to a temporary copied from the return<br>value (or its base class subobject) of the conversion function
</td>
<td> bound to the return<br>value (or its base class<br>subobject) directly
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1299.html">CWG 1299</a>
</td>
<td> C++98
</td>
<td> the definition of temporary was unclear
</td>
<td> made clear
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="initializer_list.html" title="cpp/language/constructor">constructor</a>
</li><li> <a href="converting_constructor.html" title="cpp/language/converting constructor">converting constructor</a>
</li><li> <a href="as_operator.html" title="cpp/language/copy assignment">copy assignment</a>
</li><li> <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>
</li><li> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code></a>
</li><li> <a href="initialization.html" title="cpp/language/initialization">initialization</a>
<ul><li> <a href="constant_initialization.html" title="cpp/language/constant initialization">constant initialization</a>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a>
</li></ul>
</li><li> <a href="move_operator.html" title="cpp/language/move assignment">move assignment</a>
</li><li> <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a>
</li><li> <a href="new.html" title="cpp/language/new"><code>new</code></a>
</li></ul>

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